Top sheet for absorbent articles, and method for producing it

ABSTRACT

Provided is a top sheet for absorbent articles including a number of perforations for covering a liquid-receiving surface of an absorbent article. The top sheet is formed of a thermoplastic resin containing a particulate material. The top sheet is provided with fine convex portions of the particulate material on the surface thereof and a plurality of protrusions extending from the surface thereof, and the height of each protrusion from the surface is larger than that of each fine convex portion therefrom.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an absorbent article for diapers,sanitary napkins, pantiliners, incontinence pads, etc., and to a methodfor producing a top sheet of the absorbent article. More precisely, theinvention relates to such an absorbent article of which the top sheethas a good feel, and to a method for producing the top sheet thereof.

2. Description of the Related Art

Recently, the absorbent articles are widely used for sanitary napkins,pantiliners, incontinence pads, disposable diapers and others forabsorbing excretions. Such absorbent article generally comprises aliquid-pervious top sheet to face the skin of a wearer (the top sheet isalso referred to as a body facing surface), a liquid-impervious backsheet (the back sheet is also referred to as a garment facing surface),and a liquid-absorbing layer sandwiched between the top sheet and theback sheet. Since the top sheet is directly contacted with the skin of awearer, it is desired to have a good feel. In addition, since the topsheet directly receives excretions, it is preferable that liquidexcretions hardly stay therein so that the top sheet itself hardlybecomes stuffy and sticky.

For example, Japanese Unexamined Patent Publication (Kokai) No. Heisei5-200065 discloses a surface member for absorbent articles, which has afine convex-concave pattern (this convex-concave pattern is alsoreferred to as an embossed pattern hereinafter) on the surface thereof.As having the fine embossed pattern, the surface member disclosedtherein does not have a plastic-like feel, excretions which it hasreceived can immediately pass through its surface, and it hardly becomesstuffy and sticky. Furthermore, International Unexamined PatentPublication (Kohyo) No. Heisei 9-505223 discloses a liquid-pervious,porous plastic web containing a particulate material. The web is formedinto a top sheet for absorbent articles which is soft and has a littleplastic-like feel.

However, these still have some problems. In case of the former, thesurface member is not sticky in dry, but when wetted, it shall havepowers of resistance to liquid flowing thereon owing to the embossedpattern, and results in keeping liquid in recesses of the embossedpattern on its surface. In case of the latter, the web does not becomeso sticky even when wetted, but its ability to significantly remove thesticky feel in wet is still unsatisfactory. In producing the web, theparticulate material is added to a semi-molten resin. However, the webthus produced is problematic in that the particulate material oftendrops off from its surface in use.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a top sheet forabsorbent articles, which is not sticky and has a good feel even in wet,and also to provide a method for producing such a top sheet.

According to an aspect of the invention, a top sheet may include anumber of perforations for covering a liquid-receiving surface of anabsorbent article, wherein;

-   -   the top sheet is formed of a thermoplastic resin containing a        particulate material, and    -   the top sheet is provided with fine convex portions of the        particulate material on the surface thereof and a plurality of        protrusions extending from the surface thereof, and the height        of each protrusion from the surface is larger than that of each        fine convex portion therefrom.

In this case, the particulate material has, for example, a mean particlesize of from 0.1 μm to 30 μm. Preferably, the thermoplastic resincontains at least two types of particulate material that differ fromeach other in the mean particle size by at least 9 μm. Also preferably,the amount of the particulate material to be in the thermoplastic resinthat forms the top sheet is from 20 to 150 parts by weight relative to100 parts by weight of the thermoplastic resin.

The mean particle size referred to herein is obtained by measuring themajor diameter of each particle of the particulate material thatcomprises a number of particles, followed by averaging the resultingdata.

The mean height of the protrusions from the surface of the top sheetfalls, for example, between 0.05 and 1.0 mm. The mean height is obtainedby measuring the height of each protrusion that extends from the surfaceof the top sheet, followed by averaging the resulting data.

Preferably, the top sheet has micropores that allow water vapor to passthrough it.

The protrusions may be formed by mechanically stretching the top sheet,for example, by use of needles.

According to another aspect of the invention, a method for producing atop sheet for absorbent article, may comprise;

-   -   (a) a step of mixing from 20 to 150 parts by weight of a        particulate material with 100 parts by weight of a thermoplastic        resin, followed by melt-extruding the resulting mixture to form        a sheet material, and    -   (b) a step of placing the sheet material on the surface of a        perforating member, followed by vacuuming the sheet material        through perforating holes of the perforating member to perforate        the sheet material.

Preferably, the method may further comprise, before the step (b), a step(c) of partially stretching the sheet material by use of needles to forma number of protrusions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a partially enlarged cross section ofone embodiment of a top sheet according to the invention;

FIG. 2 is a flowchart for producing the top sheet of the invention; and

FIG. 3 is a cross-sectional view of one embodiment of an absorbentarticle of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of a partially enlarged cross section ofone embodiment of a top sheet according to the invention.

As shown in FIG. 1, a top sheet 1 is made of a sheet (film) of athermoplastic resin 2 containing a number of particles 3 therein. A partof the particles 3 are exposed outside the surface of the top sheet 1.Around the particles 3, formed are micropores 6 that allow water vaporto pass through the top sheet 1. In addition, the top sheet 1 hasperforations 4 for allowing excretions to pass through them from thesurface side toward an absorbent core (a liquid-absorbing layer)generally disposed below the top sheet 1. Further, the top sheet 1 isformed with a number of protrusions 5 on the surface thereof.

The thermoplastic resin 2 for the top sheet 1 is selected from a groupgenerally consisting of polyethylene, polypropylene, polyvinyl chloride,starch base resins, polyvinyl alcohol, polyurethane, polycaprolactonecellulose esters and blends thereof.

Any materials may be used for the particles 3, provided that they arenot deformed under heat and are safe to human bodies. Among these, inorder to effectively reduce the sticky feel of the top sheet in wet,they are preferably inorganic particles of at least one type selectedfrom a group consisting of titanium oxide, calcium carbonate, soda ash,gypsum, calcium sulfate, barium sulfate, sodium sulfate, magnesiumcarbonate, magnesium sulfate, clay, calciumphosphate, silicicanhydride,carbon and talc.

Preferably, the size of the particles 3 falls between 0.1 and 30 μm. Iftheir particle size is larger than the uppermost limit of the definedrange, the particles will too much roughen the top sheet, and will givea rough feel to the skin of a wearer, and, as a result, the top sheetwill lose a good feel. On the other hand, however, if their size issmaller than the lowermost limit of the defined range, the particlescould not well roughen the top sheet to a desired degree and thereforecould not well reduce the contact area of the top sheet to the skin of awearer, and, as a result, the top sheet will have a sticky and stuffyfeel. More preferably, the size of the particles 3 falls between 0.1 and20 μm, even more preferably between 0.5 and 15 μm. It should be notedthat the particle size referred to herein is meant to indicate theaverage of the major diameter of each particle.

Also preferably, the thermoplastic resin to form the top sheet containsfrom 20 to 150 parts by weight of the particulate material relative to100 parts by weight of the resin. If the content of the particulatematerial in the resin is lower than the lowermost limit of the definedrange, it is unsatisfactory to reduce a sticky and stuffy feel of thesheet of the resin. On the other hand, however, if the content is largerthan the uppermost limit of the defined range, the strength of the sheetwill be low.

In the invention, at least two types of particulate material that differfrom each other in the mean particle size by at least 9 μm may be addedto the thermoplastic resin to form the top sheet 1. In this case,large-size particles shall exclusively roughen the top sheet 1 so thatthe sheet can have a suitable degree of surface roughness. A part oflarge-size particles and small-size particles are exposed outside thesurface of the top sheet 1 to thereby reduce the filmy feel and thesticky feel of the top sheet.

Regarding the proportion of such different types of particulate materialto be contained in the resin, it is desirable that the number ofsmall-size particles is larger than that of large-size particles. It isalso desirable that the mass of large-size particles is larger than thatof small-size particles.

One embodiment of forming the top sheet 1 that contains two differenttypes of particulate material comprises blending 1-μm particles and10-μm particles in a ratio of 40:60, followed by adding the resultingmixture to a thermoplastic resin so that the total of these 1-μmparticles and 10-μm particles could fall between 20 and 150 parts byweight relative to 100 parts by weight of the thermoplastic resin. Theresin containing the particulate material therein is then formed intothe top sheet 1.

One embodiment of forming the top sheet 1 from the thermoplasticmaterial 2 containing the particles 3 therein is shown in FIG. 2.

Thermoplastic resin pellets are firstly fed into a melt extruder 10along with particles 3, then the resin pellets are melted under heattherein, and the resin melt is mixed with the particles. The resultingresin mixture is extruded out through a T-die 11, then cooled, andbiaxially or monoaxially stretched in a stretcher 12 to be a sheetmaterial (film material). In this process, since the sheet material(film material) for the top sheet 1 is formed from the melt of the resinmixture, the particles 3 hardly drop off from the top sheet to befinally made of the sheet material.

Preferably, the maximum draw ratio in CD (cross direction) and/or MD(machine direction) of the film to obtain the sheet material fallsbetween 1.1 and 5.0 times or so. The film is stretched at leastmonoaxially, while being so controlled as to have a basis weight of from20 to 50 g/m². When being stretched to the preferred draw ratio, theresulting sheet could have the micropores 6 suitably formed around theparticles 3 as shown in FIG. 1.

As having such mircopores 6, the top sheet 1 is favorable to theabsorbent articles, since the excretions having still remained on thetop sheet 1 can infiltrate into it through the micropores 6.Accordingly, the surface of the top sheet 1 can be kept nearly dryanytime in use to reduce the unpleasant feel of the wearers. Inaddition, since the surface of the top sheet 1 can be kept nearly dry inuse, the particles 3 exposed outside the top surface 1 are not buried inexcretions, and the surface roughness of the top sheet 1 cannot bedegraded. However, if the draw ratio of the film to obtain the sheetmaterial for the top sheet 1 is smaller than the lowermost limit of thedefined range as above, the micropores 6 could not be formed in thedesired manner. If so, the excretions having remained thereon could notwell infiltrate into the top sheet 1. On the other hand, if the drawratio is larger than the uppermost limit of the defined range, theexcretions having remained thereon will too much infiltrate into the topsheet 1. If so, the top sheet 1 will feel rather wet. In case where theexcretion is the menstrual blood, the top sheet 1 will be much stainedin red with the menstrual discharges, and the appearance thereof will bebad. In addition, if the draw ratio is larger than the uppermost limitof the defined range, the sheet material will be subjected to resinorientation, and the thus-oriented sheet material will be hardlyperforated in the subsequent perforating step.

In order that the top sheet 1 to be made of the sheet material couldhave a suitable water vapor transmission rate, it is desirable that thewater vapor transmission rate of the sheet material is at most 2000cc/24 hrs/m², and more preferably falls between 50 and 1000 cc/24hrs/m².

As shown in FIG. 2, the sheet material having been stretched in thestretcher 12 is then embossed under pressure in a needle-embossing unit13. In the unit 13, the sheet material is mechanically and partiallystretched to have a number of the protrusions 5 extending from itssurface toward the skin of wearers. The protrusions 5 produce an elasticcushion-like fabric feel. When the sheet material is thus partiallystretched, the micropores will be enlarged, and, in addition, the sheetmaterial itself is further stretched and thinned, and becomes soft.However, the top sheet 1 of the invention may not have the protrusions5.

Preferably, the diameter of the respective protrusions 5 falls between0.5 and 3.0 mm. Smaller protrusions than those falling within thedefined range are difficult to form. Larger protrusions than thosefalling within the defined range, if formed, will lower the strength ofthe top sheet, and will deform the perforations 4 which are to be formedthrough the sheet in the subsequent step. Regarding the shape of theprotrusions, they may be cubic, semi-spherical, conical, quadrangularpyramid, polygon-based columnar, or polygon-based pyramidal.

Preferably, the height of the respective protrusions 5 from their bottomfalls between 0.05 and 1.0 mm. The height referred to herein is meant toindicate the average of the height of each protrusion. If their heightis smaller than the lowermost limit of the defined range, theprotrusions 5 will lose their ability to improve the feel of the topsheet to the skin of wearers. On the other hand, if their height islarger than the uppermost limit of the defined range, the protrusions 5will be readily crushed and could not keep their shape, and thereforewill also lose their ability to improve the feel of the top sheet to theskin of wearers.

In view of their feel to the skin of wearers, the distance between theneighboring protrusions 5, 5 preferably falls between 0.1 and 0.5 mm onaverage, more preferably between 0.2 and 0.3 mm.

The load in compression of the protrusions 5, i.e., a LC value thereofpreferably falls between 0.05 and 0.5. The LC value indicates thebehavior of the protrusions under compression, and is measured by use ofa texture feel tester, Katotec's KES. If their LC value is smaller thanthe lowermost limit of the defined range, the protrusions will bereadily crushed by the pressure of the body of wearers. If their LCvalue is larger than the uppermost limit of the defined range, theircompressive resistance will increase and therefore the protrusions willbe hard.

Preferably, the area ratio to form the protrusions 5 is at most 40% ofthe sheet material. If the area ratio to form the protrusions 5 islarger than the defined range, the strength of the top sheet made of thesheet material will be low. If so, in addition, the top sheet will bebroken while it is produced or while used. In order to make the topsheet have a fabric-like soft feel, the area ratio of the protrusions 5is more preferably controlled to fall between 5 and 30%, even morepreferably between 5 and 20%.

The protrusions 5 are not always required to be formed uniformly in theentire surface of the top sheet 1. They may be formed more in the areathat may be rubbed against the skin of wearers, for example, in thecrotch area than in any other area. For example, the area ratio of theprotrusions 5 may be reduced in some degree in the region correspondingto the center region of the absorbent article, and may be increased insome degree in the region corresponding to the opposite side portionsthereof.

After the protrusions 5 have been formed thereon, the sheet material isplaced on a perforating drum 14 (a perforating member), and vacuumedthrough holes of the perforating drum 14, whereby the intendedperforations 4 that correspond to the holes of the perforating drum 14are formed in the overall area of the sheet material through it(“vaccumed” means that the sheet material is to be sucked in a vacuum).The perforation density may fall between 50 and 500 perforations/cm²,but preferably between 80 and 250 perforations/cm². The mean diameter ofthe perforations preferably falls between 0.5 and 2.0 mm. The meandistance between the neighboring perforations is preferably 0.3 mm orso.

The top sheet 1 of the invention may be coated with a hydrophilicsurfactant for promoting infiltration of menstrual discharges thereinto.

FIG. 3 is a cross-sectional view of a sanitary napkin, showing oneembodiment of using the top sheet 1 in absorbent articles.

As shown in FIG. 3, the top sheet 1 covers the liquid-receiving surfaceof an absorbent core (liquid-absorbing layer) 22. The absorbent core 22may be a liquid-absorbing layer of beaten pulp and the like optionallymixed with SAP (super absorbent polymers). The absorbent core 22 is alsowrapped in a liquid-pervious sheet of tissue or the like.

On the back surface of the absorbent core 22, generally provided is aliquid-impervious back sheet 21. The back sheet 21 may be made of, forexample, a vapor-pervious but liquid-impervious polyolefin resin filmand the like. Around the absorbent core 22, the top sheet 1 and the backsheet 21 are bonded with a hot-melt adhesive or the like.

In the sanitary napkin of the illustrated embodiment, the menstrualdischarges applied to the top sheet 1 pass the top sheet 1 through theperforations 4, and are then absorbed by the absorbent core 22.

The top sheet 1 of the invention is applicable to any other absorbentarticles, in addition to the sanitary napkin of the illustratedembodiment, for example, to pantiliners, incontinence pads, disposablediapers, etc.

EXAMPLES

The invention is described in more detail with reference to thefollowing Examples, which, however, are not intended to restrict thescope of the invention.

Example 1

50 parts by weight of a particulate material, CaCO₃ (having a particlesize of 1.8 μm or 15 μm) was added to 100 parts by weight of athermoplastic resin, polyethylene (LLDPE (linear low-densitypolyethylene); having a density of 0.197 and a melt flow rate (MFR) of14), and the resulting mixture was formed into a film, which was thenstretched by 1.3 times monoaxially in MD (machine direction) to preparea sheet material. The sheet material was perforated on a perforatingdrum by vacuuming it to prepare a top sheet of the invention having abasis weight of 25 g/m².

MIU (mean coefficient of friction, μ) and MMD (mean deviation offriction coefficient) of the top sheet were respectively measured, forwhich was used a texture feel tester, Katotec's KES. The sheet notcontaining the particulate material as a comparative sample was alsoprepared, and tested in the same manner as herein.

(Result) Particle Size: 1.8 μm MIU = 0.156 MMD = 0.0142 Particle Size:15 μm MIU = 0.138 MMD = 0.0177 Comparative sample MIU = 0.185 MMD =0.0096

From the result thereof, it is understood that the MIU value of the topsheet of the invention is lowered and the MMD value thereof isincreased, whereby the top sheet become more slippery. It should benoted that the MIU value is used to indicate the surface lubricatingproperties of the sheet, and the MMD value is used to indicate thesurface roughness of the sheet. This is more significant with theincrease in the particle size of the particulate material to be added.

Example 2

50 parts by weight of a particulate material, CaCO₃ (having a particlesize of 5.4 μm) was added to 100 parts by weight of a thermoplasticresin, polyethylene (LLDPE, having a density of 0.197 and MFR of 14),and the resulting mixture was formed into a film, which was thenstretched by 1.3 times monoaxially in MD to prepare a sheet material.Using a pin plate (this has a number of small pins having a diameter of0.8 mm at regular intervals of 3.5 mm in MD and 4.0 mm in CD) combinedwith a silicone plate, the sheet material was embossed to make it have anumber of protrusions on its surface. The height of the protrusions fellbetween 0.35 and 0.55 mm. The obtained sheet was then perforated on aperforating drum by vacuuming it to obtain the top sheets of theinvention. The LC value (load in compression) and the RC value (rate ofrecovery) of the non-perforated sheet material and the perforated topsheet were measured, respectively.

(Result) Non-perforated LC = 0.204 RC = 45.1 sheet material Perforatedtop LC = 0.630 RC = 46.9 sheet

From the result thereof, it is understood that, in the sheet materialhaving been processed to have protrusions thereon and having beenfurther perforated to have perforations therethrough, the micropores areenlarged and, as a result, the sheet material becomes softer to improvethe bulk recovery properties.

As set forth above, the top sheet of the invention has a soft feel anddoes not have a plastic-like sticky and stuffy feel, though it is madeof resin. As the micropores are formed in the top sheet, it is possibleto keep the top sheet dry while used, and give no unpleasant feel to thewearers.

In addition, from the top sheet produced according to the method of theinvention, the particulate material hardly drops off.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1-7. (Cancelled)
 8. A method for producing a top sheet for absorbentarticle, comprising; (a) a step of mixing from 20 to 150 parts by weightof a particulate material with 100 parts by weight of a thermoplasticresin, followed by melt-extruding the resulting mixture to form a sheetmaterial, and (b) a step of placing the sheet material on the surface ofa perforating member, followed by vacuuming the sheet material throughperforating holes of the perforating member to perforate the sheetmaterial.
 9. The method for producing a top sheet as set forth in claim8, which further comprises, before the step (b), a step (c) of partiallystretching the sheet material by use of needles to form a number ofprotrusions.